I'm with martinsr on this one. the first thing you gotta do is get the body lines right. I usually do this without the striker in place, then there is no ability for the striker to move the door once it gets close to the closing area. adjust the door to fit the opening, then install the striker and adjust the striker to match the latch. when done, it should close without binding, and like martinsr says, the striker shouldn't be moving the door up or down, just in. sometimes I will place a long straight magnet against the body line of the door, so it overhangs the door some, and as the door swings slowly closed I watch to see how the maget edge lines up with the body line on the cab. it makes it easy to see if the door needs to go up or down to match the body lines.
got any pics of that door/cab relationship?
have fun with it,